1. Field of the Invention
The present invention relates to a lithium rechargeable battery in which a current interrupt device (CID) in the cap assembly is designed to break and interrupt the current in the battery if the battery malfunctions.
2. Discussion of the Background
Lithium rechargeable batteries are classified as either cylinder or square depending on their external shape. The cylinder type battery may include a cap assembly that interrupts the current inside the battery if the battery experiences increased internal pressure due to a malfunction of the battery, such as over charging.
Korean Patent No. 10-0357950 discloses a cap assembly for a cylinder type lithium rechargeable battery that includes a cylindrical can and a cap assembly and insulation gasket that seal the upper end of the can. The can contains an electrolyte and an electrode assembly. The electrode assembly includes a positive electrode plate, a negative electrode plate and a separator interposed between the positive electrode plate and the negative electrode plate. The positive electrode plate, the negative electrode plate and the separator are wound in the form of a jellyroll.
The cap assembly includes a safety vent, a current interrupt device (CID), a secondary protective device and a terminal cap or a cap top. The safety vent has a center portion with a protrusion that extends downward. The safety vent is aligned below the cap assembly, and the protrusion may be deformed upward by pressure generated inside the battery. A positive electrode tap or a negative electrode tap may be welded to a lower portion of the safety vent to electrically connect the positive electrode plate or the negative electrode plate with the safety vent. When either the positive electrode plate or the negative electrode plate is electrically connected with the safety vent, the other electrode plate is electrically connected with a tap or to the cylindrical can.
The CID is installed above the safety vent. Current applied to the safety vent is routed into the secondary protective device. If the internal pressure of the battery increases due to a malfunction, the protrusion of the safety vent bends upward and breaks the CID to interrupt the current.
A conventional CID includes an outer ring and a bar that extends through the center of the outer ring. An insulation printed board with a via hole is positioned on the center of the bar. A conductive layer made from a conductive metal, such as copper, is formed on the inside walls of the via hole. A conductive thin film is formed on the upper and lower surfaces of the insulation printed board. One end of the upper conductive thin film is connected with the outer ring of the insulation printed board, and the other end of the upper conductive thin film is connected with the conductive layer in the via hole. One end of the lower conductive thin film is connected with the outer ring, and the other end of the lower conductive thin film is connected with the conductive layer in the via hole, thereby electrically connecting the upper conductive thin film with the lower conductive thin film through the conductive layer in the via hole.
The bar is provided at both its lateral portions with a breakage part adjacent to the via hole so that the bar can be easily broken.
The CID is aligned so that the safety vent protrusion may pass through and contact the via hole. When the protrusion protrudes upwards, it breaks the bar of the CID about the center of the via hole and breaks the electrical connection between the conductive layer in the via hole and the upper conductive thin film or the lower conductive thin film. Accordingly, the CID interrupts the current between the safety vent and the secondary protective device.
In conventional CIDs, the via hole is formed at the center portion of the cross bar so that the via hole breaks when the internal pressure of the lithium rechargeable battery increases. This requires the conductive layer in the via hole to be completely separated from either the upper conductive thin film or the lower conductive thin film to interrupt the current when the bar is broken by the protrusion of the safety vent. However, the conductive layer may not completely separate from the upper conductive thin film or the lower conductive thin film even if the cross bar is broken because the conductive layer is made from a flexible metal, such as copper. Therefore, even if the bar breaks, the conductive layer in the via hole may maintain electrical contact with the upper conductive thin film and the lower conductive thin film. If the current is not interrupted by the CID, the pressure in the battery may increase and cause an explosion.